Electrostatic chucks have an electrode, and the electrostatic chucks are used to hold a wafer to a wafer stage by Coulomb's force acting between the electrode and the wafer. As against mechanical chucks, in the electrostatic chucks, almost the whole of the bottom surface of the wafer closely adheres onto the wafer stage so that the wafer can be cooled effectively. Therefore, the electrostatic chuck is used widely. There are two types of electrostatic chuck: monopole electrostatic chuck and bipolar electrostatic chuck. In the case of the monopole electrostatic chucks, adsorption does not occur if there is no plasma around. Therefore, the monopole electrostatic chuck can be used only in plasma processing units. On the other hand, in the case of the bipolar electrostatic chucks, plasma is not required for the adsorption.
FIG. 5 is a top view illustrating a pattern of electrodes in a conventional electrostatic chuck. An electrostatic chuck 500 includes a pair of electrodes 501a and 501b (electrode pattern) in the form of concentric arcs. These electrodes are embedded inside an insulating body on a surface. One electrode 501a is a positive electrode and the other electrode 501b is a negative electrode. These electrodes 501a and 501b make up the whole of the wafer adsorption area of the electrostatic chuck 500. The positive electrode 501a includes a linear portion 502a formed in radial direction and a plurality of concentric arc portions 503a branched out from the linear portion 502a. The linear portions 502a and the arc portions 503a are disposed in such a way that they form a circle together. The electrodes 501a facing each other in two halves of the circle are connected to each other in the outermost peripheral circle.
The negative electrode 501b is structured similarly. However, the electrodes 502b facing each other in two halves of a circle are connected to each other at the center of the circle. The positive electrode 501a and the negative electrode 501b are formed by the arc portions 503a and 503b, which are disposed alternately and engaged like teeth of a comb. This pattern of electrodes 501a and 501b can be formed with precision by photo fabrication. When a voltage is applied on both of the positive and negative electrodes 501a and 501b of the electrostatic chuck 500, a stable and uniform adsorptivity is generated over almost the whole of the wafer adsorption area due to the electrode patterns 501 and 502b. 
Sometimes, the wafer stage employing the electrostatic chuck 500 is provided with lifting pins to separate the wafer. The lifting pins are provided such that they protrude out from lifting pinholes that are spread evenly on the wafer stage. It is desirable to have 3 lifting pins (not shown in the diagram) 120 degrees apart to ensure that the wafer is supported firmly by the lifting pins.
However, in the electrostatic chuck 500, the linear portions 502a and 502b in the radial direction of the positive and negative electrodes 501a and 501b, are 90 degrees apart. Therefore, one or two lifting pinholes have to be provided on these linear portions 502a and 502b. However, it is desirable that electrode pattern must be designed to avoid this. In one of the structures, the electrode patterns 501a and 501b that are in the form of branches are disposed opposite to each other and connected to each other at either the outermost peripheral circle or at the center. This structure results in complicating the electrode patterns 501a and 501b. 